When the three goddesses Athena, Hera and Aphrodite appeared to Paris and asked him to make a decision on who was the most beautiful goddess. As Paris was unable to decide, the goddess each offered him a gift in return if he chose them. Athena offered all the wisdom in the world as well as good swordsmanship. Hera offered him power over the nations. Aphrodite offered to give him the most beautiful woman in the world, Helen. After listening to that, Paris immediately chose Aphrodite.
So after an extencive affair with Paris she was taken to Troy which enraged Sparta. This is what set off the Trojan war.